Rotary drag devices

ABSTRACT

The disclosure relates to a rotary drag device having relatively rotatable members one ( 4 ) of which has a stack ( 5 ) of closely spaced, thin-walled flexible annular plates ( 20 ) peripherally mounted thereon to provide annular running faces and a viscous fluid filling the spaces between the plates, and the other ( 6 ) of which members has a stack ( 8 ) of segmental plates ( 33 ) having part-annular running faces interleaved with the annular plates to slide between the plates with relative rotation of the members. Slits ( 26, 27 ) in the annular plates permit release of viscous fluid from between the plates as fluid pressure rises ahead of the segmental plates with increasing relative speed between the members. The flexibility of the annular plates allowing the plates to close together accommodating the loss of viscous fluid from between the plates whereby the damping characteristic of the device can vary between viscous drag between the relative running surfaces of the annular and segmental plates at low relative speeds of the members and, with loss of viscous fluid from between the plates, can change to boundary lubrication between the running surfaces of the respective plates at higher relative speeds of rotation of the members.

This invention relates to a rotary drag device for providing a couplinghaving a selected speed/torque characteristic between two relativelyrotatable members. The invention is particularly applicable to TV/videocamera mountings such as pan and tilt heads.

Devices which produce drag or damping by means of either friction orviscous shear between relatively moving elements are well known. Theformer typically comprise a friction clutch attached to one element andcontacting a relatively moving element under pressure such that energyis dissipated by the resulting sliding contact. The magnitude of thedamping force is dependent upon the friction contact pressure and istherefore readily varied.

The torque/speed characteristic of a friction damper may be modified bythe choice of friction materials and/or the provision of a fluid at thefriction surface to provide boundary lubrication of the sliding contactin which case the damping force may rise with increasing speed from astationary position to an initial level and thereafter remainsubstantially constant irrespective of speed.

Viscous dampers typically comprise multiple interleaved platesalternately attached to each of the relatively moving elements, eachplate being separated from adjacent plates by a small gap occupied by aviscous fluid. Relative movement of the plates causes viscous shear totake place in the fluid. The damping force generated tends to increasesubstantially linearly with speed but at rest is zero.

A particularly important application of a drag/damper device is in panand tilt heads for cameras, especially television cameras. In thisapplication it is advantageous, to avoid “jerking” when making very slowcamera movements from rest, for the torque to be substantially zero atnear zero speed. On the other hand, for increasingly fast cameramovements above a certain speed, it is advantageous for the resistancetorque not to rise in proportion necessitating application of anincreasing force to move the camera.

U.S. Pat. No. 4,955,568 discloses a camera mounting pan and lift head,including fluid drag systems or resisting pan and tilt movement and acounterbalance system in supporting the camera in equilibrium in anyposition in its range of tilt movement. The fluid drag systems eachinclude a drag disc assembly secured to the rotating shaft and dragsection assemblies pivotally mounted on the housing and disposed aboutthe circumference of the disc assembly to define the space containingdiscus fluid. The level of drag exerted may be adjusted by pivoting thesector assemblies towards or away from the disc assembly to vary thespace containing viscous fluid. In this system is the drag provided willrise more or less linearly with relative rotational speed of thecomponents of the drag system so that at very high speed, there is highdrag force.

In applications such as a camera mounting, where the purpose of thedamping device is to enhance the smoothness and control of manualmovements, the following characteristics are therefore desirable:

1. The rate of change of the damping/speed relationship should tend tozero at high speed to permit relatively fast movements to beaccomplished without unreasonable restraint, whilst providing adequatedamping control at low speeds.

2. For fine control at the commencement of movement and duringreversals, the unit should produce zero damping at rest and a high rateof change of damping with speed at low speed.

Our UK Patent Application number 9018312.0 describes and illustrates arotary damper comprising two torque resisting rotary damping devicescoupled together in series, one of said rotary damping devices having aninput, an output and a viscous fluid coupling to provide a resistancetorque to relative rotation between the input and output which is zerowhen the relative speed between the input and output is zero and whichrises generally linearly with relative speed between the input andoutput. The other of the rotary damping devices has an input and anoutput and a torque resistant friction coupling which resists relativerotation between the input and output up to a certain torque and abovewhich the friction coupling allows slippage with a substantiallyconstant torque resistance. Thus the two rotary damping devices togetherprovide a combined speed/torque characteristic in which the resistancetorque generated by the viscous coupling rises with speed to a certainlevel at which the torque causes the friction coupling to slipwhereafter the resistance offered by the damper remains substantiallyconstant for higher speeds.

Providing separate friction and viscous couplings in series does howeverresult in a complicated arrangement which is difficult to pack into arestricted space. The space available for drag devices in a cameramounting is normally limited and it is highly desirable therefore tomake the drag devices as compact as possible.

This invention provides a rotary drag device having relatively rotatablemembers one of which has a plurality of closely spaced, thin-walledflexible annular plates peripherally mounted thereon to provide annularrunning faces and a viscous fluid filling the spaces between the plates,and the other of which members has a plurality of segmental elementshaving part-annular running faces interleaved with the annular plates toslide between the plates with relative rotation of the members and meansto permit release of viscous fluid from between the plates as fluidpressure rises ahead of the segmental plates with increasing relativespeeds between the members, the flexibility of the annular platesallowing the plates to close together accommodating the loss of viscousfluid from between the plates whereby the damping characteristic of thedevice can vary between viscous drag between the relative runningsurfaces of the annular and segmental plates at low relative speeds ofthe members and, with loss of viscous fluid from between the plates, canchange to boundary lubrication between the running surfaces of therespective plates at higher relative speeds of rotation of the members.

In one arrangement according to the invention the means to permitrelease of viscous fluid between the plates includes apertures in theannular plates outside the running surfaces thereof for the viscousfluid to flow axially between the plates.

More specifically the apertures may comprise elongate arcuate slitsspaced around the annular plates outside the running surfaces thereof.

In one particular arrangement according to the invention the elongatearcuate slits may comprise inner and outer sets of arcuate slitsstaggered with respect to each other to provide flexible beam elementsbetween the respective ends of the inner and outer slits to allow therunning surface part of the annular plate to move axially with respectto the peripheral mounting of the plate whereby the spacing between therunning surfaces of the annular plates can vary with the amounts ofviscous fluids between the plates.

In any of the arrangements referred to above the segmental elements mayhave means to permit limited axial movement of the running faces of theelements to accommodate axial movement of the running faces of theannular plates.

For example, the means in the segmental plates to permit axial movementof the running faces thereof may comprise inner and outer rows ofannular slits, outside the running faces of the plates with the slits inthe outer set staggered with respect to the slits in the inner set toprovide beam elements between the respective ends of the slits which canflex to allow limited axial movement of the running faces of thesegmental elements with respective to their respective mountings.

In any of the above arrangements the annular plates may be mounted onsaid one member of the device around their outer peripheral edges andthe segmental elements are mounted on said other member within theannular plates.

Also in any of the above arrangements means are provided for adjustingthe overlap between the running faces of the segmental elements and theannular plates to vary the degree of drag provided.

More specifically the segmental elements may be mounted in pairs toengage between respective pairs of annular plates.

In the case where means are provided for adjusting the overlap betweenthe running faces of the segmental and annular plates to vary the degreeof drag provided the adjusting means may comprise meas for varying thespacing of the segmental plates in each pair.

More specifically said other member of the drag device may comprise ahub having a central axis co-axial with the axis of the annular platesand a pair of segmental plate carriers slidably mounted on the hub formovement towards and away from the central axis, said pairs of segmentalplates being mounted on the outer sides of the plate carriers and adrive mechanism being provided for adjusting the spacing of the carriersand thereby the degree of overlap of the running surfaces of thesegmental plates and the annular plates.

In one specific arrangement according to the invention the mechanism foradjusting the spacing of the mounting members comprises parallelelongate toothed rack members extending from the respective platecarriers across the hub and a rotatable pinion drive engaging the racksto adjust their respective positions.

The elongate toothed rack members may be in sliding engagement with oneanother and means may be provided for taking out any play or backlashbetween the members.

In the latter arrangement the anti-backlash means may comprise a springloaded clamping arrangement acting between the hub and one of the racksmembers for holding the rack members together through the hub.

The following is a description of some specific embodiments of theinvention, reference being made to the accompanying drawings in which:

FIG. 1 is a plan view of a rotary viscous drag device controllingpan/tilt movement of a TV/video camera mounting including an outerhousing in which the viscous coupling arrangement is enclosed and acontrol mechanism for varying the degree of resistance provided by thedevice;

FIG. 2 is a front elevation view of the device showing the circularhousing and rotary control knob for varying the coupling resistance;

FIG. 3 is a rear elevation view of the device;

FIG. 4 is an elevation view of the components providing the viscouscoupling of the drag device comprising a stack of annular plates for thestator of the coupling and a stack of pairs of segmental plates for therotor with segmental plates shown in the minimum resistance positions;

FIG. 5 is a similar view to that of FIG. 4 showing the segmental platesin their maximum resistance positions;

FIG. 6 is a perspective view of part of the stack of outer and innerplates and inner pairs of segmental plates forming the viscous coupling;

FIG. 7 is a perspective view of the mounting mechanism for the innersegmental plate for adjusting the separation of the plates and includingan outer hub containing a chassis and plate carriers slidably mounted onthe chassis;

FIG. 8 is a similar view to that of FIG. 7 with the chassis removed fromthe hub;

FIG. 9 is a side view of the chassis as shown in FIG. 8;

FIG. 10 is a perspective view of the pair of plate carriers removed fromthe chassis;

FIG. 11 is a perspective view of the pair of plate carriers in theposition of maximum separation;

FIG. 12 is a similar view to that of FIG. 11 with the plate carriers intheir position of minimum separation;

FIG. 13 is a similar view to that of FIG. 11 illustrating the directionof a spring force on one of the carriers to minimise backlash in thearrangement; and

FIGS. 14 and 15 show a modified form of the device of FIG. 1.

Referring firstly to FIG. 1 of the drawings, there is shown a rotarydrag device intended, in particular, for use on a TV/video cameramounting. Two such devices are provided on the camera mounting fordamping pan and tilt movements of the mounting respectively. The deviceillustrated is particularly suitable for controlling tilt movement.

The device comprises a circular base plate 1 to one side of which thereis an enclosed circular housing 2 containing a viscous fluid coupling 3including a stator 4 consisting of a stack 5 of spaced annular platesmounted in the housing and a rotor 6 rotatably mounted about an axis 7on the base plate. The rotor carries a stack 8 of spaced pairs ofsegmental plates located within the stator with the pairs of platesinterleaved between the annular plates of the stator as will bedescribed in detail later.

The spacing of the pairs of segmental plates can be adjusted to vary thedegree of overlap of the plates with the annular plates and thereby varythe viscous drag provided by the coupling. A mechanism 9 for varying thespacing of the plates is located on the opposite side of the base plateto the viscous coupling. The mechanism comprises a central shaft mountedon the base plate in a fixed hub 10 and carrying a drive pinion 11. Toone side of the hub there is a sub-housing 12 which is bolted to thehousing 11. A shaft 13 is mounted in the housing with a pinion 14 fixedto the shaft within the housing supported in a block attached to thebase plate. The housing has a side slot 15 through which pinion 11projects to engage pinion 14. One end of shaft 13 projects from thesub-housing block and a control knob 16 is mounted on the projecting endof the shaft. The manner in which rotation of the shaft adjusts thesegmental plates will be described below.

Reference is now made to FIGS. 4 to 6 of the drawings which show therotary viscous coupling of the drag device in greater detail. The statorof the coupling comprises a stack of thin walled annular flexible metalplates 20 spaced apart at their outer peripheries by narrow annularspacer plates 21. Plates 20 have bolt holes 22 and the plates 21 havecorresponding bolt holes 23 to receive through bolts (not shown) whichsecure the stack of plates together and secure the stack in the housing.

Each annular plate 20 has inner annular running faces 24 on either sideof the plate and beyond the running faces 24 the plate has an annularband 25 in which a series of inner and outer arcuate slits 26, 27 areformed in the plate. The inner set of slits 26 is staggeredsymmetrically with respect to the outer set of slips to form arcuatebeams 28 between the ends of the adjacent inner and outer slits. Thebeams 28 can flex to allow the inner annular portion of the platecarrying the running faces 24 to flex axially with respect to the outerperiphery of the plate where it is mounted. The purpose of the flexingof the plate will be described below.

The rotor of the coupling 30 and comprises a pair of segmental platecarriers 31, 32 each of which carries a stack of thin walled flexiblesegmental plates 33 which are spaced apart by narrow spacer plates 34.The segmental plates and spacers are secured to the carriers by bolts,the holes for which are illustrated at 35.

The segmental plates 33 are interleaved between the annular plates 20 ofthe stator and have outer annular running faces 36 to engage with therunning faces 24 of the annular plates.

The segmental plates each have a part annular band 37 between therunning faces 36 and the plate mounting in which rows of inner and outerslits 38, 39 are formed. The inner slits are staggered symmetricallywith respect of the outer slits to form equal length annular beams 40between the respective ends of the slits. The arrangement enables theouter portion of each plate to flex axially with respect to its mountingin a similar manner to the way in which the annular plates flex asdescribed above.

The housing 2 is filled with a viscous fluid to penetrate the spacesbetween the annular plates 20 and to provide a film of fluid between theadjacent running surfaces of the segmental plates 33 and annular plates20 as the rotor turns in the stator. The viscous drag provided will bedetermined by the degree of overlap of segmental plates with the annularplates and this varies between the minimum plate engagement positionshown in FIG. 4 in which just the tips of the segmental plates overlapand the maximum plate engagement position shown in FIG. 5 in which thefull running faces of the segmental plates engage with those of theannular plates. The segmental plates may be tapered in cross-sectiontowards their outer edges to facilitate penetration between the annularplates as the plates are moved between the minimum and maximumpenetration positions.

It will be understood that as the rotor rotates within the statorpressure will be generated in the viscous fluid between the annularplates 20 ahead of the segmental plates with a corresponding drop inpressure behind the segmental plates. The rise in pressure ahead of theplates will tend to force fluid through the slits 26, 27 towards theends of the stack where there are cavities between the stack and sidewalls of the housing 2. The drop in pressure behind the trailing ends ofthe segmental plates causes the plates to be drawn together which ispermitted by the flexible bands 25 in the annular plates and similarflexible bands 37 in the segmental plates referred to above. As thespeed of the rotor rises, so more and more viscous fluid is ejected frombetween the plates until the viscous fluid provides only a boundarylubrication between the running surfaces of the segmental and annularplates. At that stage, the drag characteristic provided by the couplingchanges from a linear characteristic in which drag rises proportionallywith speed when there is a full viscous layer between the workingsurfaces of the segmental and annular plates to a constant drag withonly boundary lubrication between the running faces of the plates.

The mounting mechanism embodied in the rotor of the coupling foradjusting the spacing of the pairs of segmental plates will now bedescribed with reference to FIGS. 9 to 13. The mechanism comprises anouter hub having an annular wall 41 within which a generally cylindricalchassis 42 is mounted. The plate carriers 31, 32 are slidably mounted inthe chassis for movement towards and away from each other and as bestseen in FIG. 10, each plate carrier has an elongate leg extending fromthe back of the carrier across the hub in sliding engagement with theother carrier. The adjacent faces of the legs are formed with toothedracks disposed opposite one another which engage a common pinion on theshaft 13. Rotation of the shaft 13 by the knob 16 adjusts the spacing ofthe carriers 32 through the racks 44 between maximum and minimumseparation positions as shown in FIGS. 12 and 13.

In order to remove play between the carriers the carriers are pressedtogether by a spring loaded strip 50 having an inclined face on one sidewhich bears against a corresponding inclined face on an outer side of aleg of one carrier. The other side of the strip bears against theadjacent inside wall of the chassis.

The leg of the other carrier member also has an inclined face whichbears against an adjacent inclined face on the chassis. The action ofthe spring loaded strip is to hold the carrier members together toeliminate any play or backlash. The sprung strip 50 has bores indicatedby arrows on FIG. 13 to receive spring loaded screw adjustors (51, 52)mounted on the base plate of the housing 1 to engage the strip 50. Thespring pressure can be adjusted to take up any wear in the carriermembers by adjusting the screw adjusters 52.

It will be appreciated that many modifications may be made to the abovedescribed embodiment without departing from the scope of the invention.For example, the control rotor of the device could be held stationaryand the stator rotated with respect to it. Such an arrangement could beutilised for controlling pan movement of the camera mounting and couldbe provided with a bevel gear drive for the control knob of themechanism so that the knobs for controlling both pan and tilt can bearranged side by side one another on the camera mounting.

Furthermore, the drag characteristics of the running surfaces of theannular plates and/or segmental plates may be modified in the boundarylubrication mode by surface treatments/coatings applied to the surfaces.For example the surfaces may be coated with a dry lubricant such aspolytetrafluoroethylene (P.T.F.E.).

Preferably the cavity of the rotary drag device is sealed off fromatmosphere to prevent loss of viscous fluid from the device. However asealed drag unit which is not completely filled with fluid could sufferfrom a loss of drag output due to the presence of air bubbles betweenthe adjacent plates, reducing the effective working area of theoverlapping plates.

A means of overcoming this problem is to immerse the drag plates in asealed volume of viscous fluid, initially evacuated to ensure that noair is present. However a sealed unit of this nature requires a meansfor accommodating the change in volume of the viscous fluid with changesin temperature.

In one specific arrangement according to the invention, the means foraccommodating changes in temperature in the viscous fluid comprises athin-walled bladder immersed in the viscous fluid and vented at one ormore places to atmosphere outside the drag unit. The bladder is flexibleenough to change volume without storing enough energy to cause asignificant change in pressure in the viscous fluid.

Referring to FIG. 14, there is shown a sealed version of the rotary dragdevice of FIG. 1 and provided with a thin-walled bladder to accommodatechanges in the viscous fluid temperature. The base plate 1 of the devicehas a central circular boss 60 on which the rotor 6 is mounted. The bosshas an encircling seat 61 on which an inner race of a radial bearing 62is supported and an outer race for the bearing is mounted in a seat 63formed in an extension 64 of the stator 4. A dynamic seal 65 is mountedin a seat 66 in the stator adjacent bearing 62 to seal with the boss 60and thereby close off the cavity containing the viscous fluid couplingto atmosphere to prevent loss of fluid.

The stator 4 is formed in two parts as before but in this arrangementthe stator end cap 67 has an encircling integral sleeve 67 a whichengages in a recess 68 in the outer side of the stator ring 4 with astatic seal 69 mounted between the sleeve and ring to prevent loss offluid between them. The end cap is secured to the ring by bolts 70 asbefore but in this case the bolts pass through and secure the stack ofannular plates 5 between the end cap and ring.

The bolts also secure an annular plate 71 in the enclosure of thedevice. One side of the plate and on opposite face of the stator ring 68are formed with part circular cross-section recesses 72, 73 respectivelyto receive between them an annular thin-walled bladder 74 which isvented through a port or ports to atmosphere.

In the arrangement shown in FIG. 15, the bladder takes the form of athin-walled tube connected to a T-piece 75, the T-piece being vented toatmosphere through a port hole 76 in the wall of the stator sealing ring77 prevents the viscous fluid entering the T-piece and bladder.

What is claimed is:
 1. A rotary drag device having first and secondmembers rotatable with respect to one another, the first member having aplurality of closely spaced, thin-walled flexible annular platesperipherally mounted thereon forming annular first running faces and aviscous fluid filling the spaces between the annular plates, the secondmember having a plurality of segmental elements forming part-annularsecond running faces interleaved with the annular plates to slidebetween the annular plates upon relative rotation between the first andsecond members, further comprising fluid release means permitting arelease of viscous fluid from between the annular plates as a fluidpressure rises ahead of the segmental elements as a relative speedbetween the first and second members increases, wherein, when therelative speed between the first and second members is below apredetermined speed, the rotary drag device operates in a viscouscoupling mode where a drag force rises proportionally with increasingspeed up to the predetermined speed and where the annular plates flex toallow the annular plates to approach one another accommodating for aloss of viscous fluid from between the first and second running facesvia the fluid release means and, when the relative speed is at least thepredetermined speed, the rotary drag device operates in a boundarylubrication mode where the drag force is substantially constant withincreasing speed over the predetermined speed and where a reduced amountof viscous fluid remaining between the annular plates provides boundarylubrication between the first and second running faces.
 2. A rotary dragdevice as claimed in claim 1, wherein the fluid release means includesapertures in the annular plates outside the first running facespermitting the viscous fluid to flow axially between the annular plates.3. A rotary drag device as claimed in claim 2, wherein the aperturescomprise elongate arcuate slits spaced around the annular plates outsidethe first running faces.
 4. A rotary drag device as claimed in claim 3,wherein the elongate arcuate slits comprise an inner set and an outerset of arcuate slits, the inner arcuate slits being staggered withrespect to the outer slits to provide flexible first beam elementsbetween respective ends of the inner and outer slits to allow portionsof the annular plates forming the first running face to move axiallywith respect to peripheral mounting portions thereof without relativerotation whereby a spacing between the first running faces can vary witha change in the amount of viscous fluid between the annular plates.
 5. Arotary drag device as claimed in claim 1, wherein the segmental elementshave means permitting limited axial movement of the second running facesto accommodate axial movement of the first running faces.
 6. A rotarydrag device as claimed in claim 5, wherein the means permitting axialmovement includes a row of inner annular slits and a row of outerannular slits outside the second running faces with the outer slitsstaggered with respect to the inner slits to provide second beamelements between respective ends of the inner and outer slits, thesecond beam elements flexing to allow limited axial movement of thesecond running laces with respect to mountings thereof.
 7. A rotary dragdevice as claimed in claim 1, wherein the annular plates are mounted onthe first member around outer peripheral edges thereof and the segmentalplates are mounted on the second member within the annular plates.
 8. Arotary drag device as claimed in claim 1, further comprising overlapadjusting means for adjusting an overlap between the second runningfaces and the annular plates to vary a degree of drag provided.
 9. Arotary drag device as claimed in claim 8, wherein the segmental elementsare mounted in pairs and each pair of segmental elements is engagedbetween two adjacent annular plates.
 10. A rotary drag device as claimedin claim 9, wherein the overlap adjusting means includes means foradjusting a spacing between the segmental elements of each pair.
 11. Arotary drag device as claimed in claim 10, wherein the second membercomprises a hub having a central axis co-axial with an axis of theannular plates and a pair of segmental element carriers slidably mountedon the hub for movement towards and away from the central axis, thepairs of segmental elements being mounted on outer sides of the elementcarriers and a space adjusting drive mechanism for adjusting a spacingof the element carriers, the space adjusting drive mechanism therebyadjusting the overlap of the first and second running faces.
 12. Arotary drag device as claimed in claim 11, the space adjusting drivemechanism comprises a plurality of parallel elongate toothed rackmembers, each rack member extending from a respective element carrieracross the hub and a rotatable pinion drive engaging the rack members toadjust their respective positions.
 13. A rotary drag device as claimedin claim 12, further comprising anti-backlash means for taking out playbacklash between the rack members and the element carriers.
 14. A rotarydrag device as claimed in claim 13, wherein the anti-backlash meanscomprises a spring loaded clamping arrangement acting between the huband one of the rack members for holding the rack members togetherthrough the hub.
 15. A rotary drag device as claimed in claim 1, whereinat least one of the first and second running faces have one of surfacetreatments and coatings to modify drag coefficients thereof when theviscous fluid provides boundary lubrication between the first and secondrunning faces.
 16. A rotary drag device as claimed in claim 15, whereinthe at least one of the first and second running faces have a P.T.F.E.coating.
 17. A rotary drag device as claimed in claim 1, furthercomprising sealing means between the first and second members to sealoff a space containing the annular plates and the segmental elements toprevent loss of viscous fluid, wherein a flexible walled bladder ismounted in the space and is vented to atmosphere to accommodateexpansion and contraction of the viscous fluid with change intemperature.